Magnetic position sensors are extremely important devices. They are used in the automotive industry to sense the angular position of a shaft such as the crank or cam shaft of an engine. Information on the shaft position is then used for fuel and ignition timing. In order to meet the On Board Diagnostic II misfire detection mandate, a very high degree of angular accuracy is required from the position sensor. An illustrative method and apparatus for accurately determining camshaft position is shown in U.S. Pat. No. 5,570,016, assigned to the assignee of this invention. A similar method and apparatus for a position sensor in an automotive steering system is shown in U.S. Pat. No. 4,924,696, also assigned to the assignee of this invention.
Briefly stated, the methods use two magnetic field sensors, a bias magnet, and a magnetically soft, ferromagnetic target wheel with two distinct target tracks or regions. A schematic diagram of a representative arrangement is shown in FIG. 1 of this specification. The permanent bias magnet spans and underlies the backside of both closely-spaced magnetic field sensors and opposes both target regions. The field generated from a first underlying region of the bias magnet couples strongly to the opposing region of the target and modulates the output of the first overlying magnetic field sensor. The field generated from a second underlying region of the bias magnet couples strongly to the opposing region of the target and modulates the output of a second magnetic field sensor. Cross talk between the two sensors is controlled by their physical separation. The second target track may be a mirror image of the first, providing maximum accuracy or simply a constant pattern for use as a reference signal for temperature and air gap fluctuation corrections.
In one automotive application of this technology to crankshaft angle sensors, thin film semiconductor magnetoresistors (mr's) are used as the magnetic field sense elements. This choice leads to a reasonably robust modulation of the sensor voltage and straightforward processing electronics. In practice the magnetoresistors are powered by matched current sources. The voltages generated at the two magnetoresistors are given by: EQU v.sub.1 =i.sub.1 .times.mr.sub.1 EQU v.sub.2 =i.sub.2 .times.mr.sub.2
These voltages are input to a comparator circuit which converts the voltage modulation to a digital signal for use by an engine controller. The target wheel chosen has a series of long and short teeth in one track and a toothless (constant diameter) structure in the second track. Typical voltage outputs from the magnetoresistors are shown in FIG. 4. It is seen that the transition from a region of a tooth (high voltage) to a space (low voltage) has a distinct slope. The fact that this transition is not perfectly abrupt is related to the source of error in the application. If the quiescent resistance of the magnetoresistors is not well matched to one another, this will shift the switching point of the comparator.
The key to gaining the required accuracy is to improve the matching of the magnetoresistors. The mr's have an inherent mismatch due to fluctuations in their manufacture. In addition to this, the resistance of the devices will vary because of subtle differences in their bias field. The differences in the bias field come from fluctuations in the composition of the bias magnet, irregular magnetization of the bias magnet, or errors in locating the mr's with respect to the magnet's surface (i.e., devices off-center, or skewed to different heights above the surface). If one can independently adjust the bias field seen by one of the two magnetoresistors, the devices could be matched to any precision desired. This can compensate for all sources of errors: resistance fluctuations, magnet irregularities, and tolerance problems.